Transportable lift truck with telescopic lifting arm

ABSTRACT

A self propelled transportable lift truck (A) having a frame ( 1 ) formed by two spaced parallel longitudinal front members and a rear transverse member, at least two front wheels ( 5, 6 ) and at least one steerable rear wheel ( 7 ), such that the lifting mechanism thereby extends from a most retracted position between the longitudinal members of the frame. The lifting mechanism to raise and/or lower the lifting tines comprises a telescopic lifting arm ( 4 ) articulated about a horizontal axis perpendicular to the longitudinal axis of the lift truck. An operator&#39;s cab ( 2 ) is located to one side of the lifting mechanism; the motive drive unit ( 3 ) is housed beneath the lifting mechanism. The lifting mechanism is provided with a yaw control to facilitate the lateral movement of the lifting lines. In addition, the yaw control has, on demand, the capability to automatically self centre the lifting mechanism. The lifting mechanism is further provided with a control system ( 52, 53, 54 ) which enables an approximate straight line lift to prevent tipping when lifting the lift truck&#39;s maximum rated load. The lift truck is provided with an automatic system ( 42, 43, 45 ) for positively captivating itself when mounted and transported on the back of a carrier vehicle such as a truck or trailer. The lift truck is capable of being used on both paved and unpaved (rough) ground.

The present invention relates generally to the field of self propelledlift trucks. More particularly, it relates to the transportable varietywhich require an extremely low unladen weight relative to its loadcarrying capacity.

There are many types of lift trucks designed for material handlingapplications.

A number of such lift trucks use piston-cylinder arrangements orchain-link drive mechanisms combined with sprocket wheels to raiseand/or lower the lifting tines on a vertical mast. Others use anarticulating linkage and gantry structure to raise and/or lower thelifting tines. While, yet others, use a telescopic lifting armarticulated about a horizontal axis perpendicular to the longitudinalaxis of the lift truck to raise and/or lower the lifting tines. Forexample, such vehicles are disclosed in U.S. Pat. Nos. 3,858,730;4,345,873; 4,365,921; 4,531,615; 4,621,711; 4,826,474; 4,921,075;4,986,721; 5,061,149; 5,199,861; 5,478,192; D384,477; European PatentSpecification 0 701 963 B1, and Irish Patent Specification S75661.

The materials handling lift truck of the present invention is adapted tobe securely mounted and transported on the back of a carrier vehicle,such as a trailer or truck. In this manner, the lift truck can beconveniently transported directly to the work site, along with the loadto be moved.

For a lift truck to be effectively used in this manner, it should belightweight, and yet at the same time, should be capable of liftingheavy loads. Form following function dictates that to provide heavylifting capability while minimizing the weight of the lift truck, it isadvantageous for the lifting tines to have the capacity to be shiftedfrom a position forward of the front wheels to a position behind themthereby relocating the centre of gravity of the load towards the centreof gravity of the vehicle and thus increasing the lifting capacity ofthe lift truck. For this reason, the aforementioned type lift truckgenerally has a U-shaped frame with the open end toward the front, atleast two front wheels and at least one steerable rear wheel, such thatthe lifting device thereby extends from a most retracted positionbetween the lateral members of the U-shaped frame.

The prior art also discloses lift trucks which utilize heavycounterweights thereby increasing the load handling capability of thelift truck. Such mechanisms are inconsistent with the need to minimizethe weight of a lift truck designed to be transported on the back of acarrier vehicle, since such counterweights reduce the useful payloadcapability of the carrier vehicle as well as increase the moment loadapplied at the rear of the carrier vehicle.

Prior art lift trucks may avoid the need to use heavy counterweights bypositioning the lifting tines and the load they carry between the frontand the rear wheels. However, this type of design has been generallylimited to applications where the load to be lifted is at ground level.For example, such vehicles are disclosed in U.S. Pat. Nos. 3,610,453 and3,861,535.

In general, all lift trucks of the prior art, capable of beingtransported on carrier vehicles have the disadvantage of obstructing thefield of view from the operator's station.

The transportable lift trucks of the prior art generally have minimalground clearance and are not as capable of travelling over rough orbumpy terrain, but are more suited to paved ground.

A feature that some lift trucks of the prior art provide is a“sideshift” mechanism. That is, the ability to move the tine carriageand lifting tines a fixed distance left or right of the longitudinalaxis of the lift truck thus providing the vehicle operator the abilityto manipulate the pickup or drop off of a load without having toreadjust the position of the whole vehicle which would typically involvereversing/driving forward the vehicle in order to renegotiate a moreoptimum load approach. The method in which, and distance that the liftedload can be shifted from the longitudinal axis of the lift truck iscritical on transportable type lift trucks since their function dictatesa compact and light design.

In the case of prior art trucks that utilize an integrated pivot and“sideshift” assembly, namely those that utilize a telescopic lifting armarticulated about a horizontal axis as the lifting device, the liftingdevice thus having the capability of being displaced transversely in thehorizontal plane as well as being articulated in the vertical plane, hasthe disadvantage, in that, with the lifting device and “sideshift”structure assembly being shifted transversely in the horizontal plane,there results a significant shift laterally of the lift truck's centreof gravity; such a shift thus leading to a notable reduction in thepermissible amount of “sideshift” in order to maintain vehiclestability. Furthermore, this arrangement has the disadvantage in that itinvolves high fabrication costs and significant maintenance costs inexchange for a relatively minimal guarantee of functional reliability.

In the case of prior art trucks that have a “sideshift” mechanism orsystem that connects the vertical mast to the tine carriage supportingthe lifting tines, or the telescoping lifting arm to the tine carriagesupporting the lifting tines; this system has the disadvantage ofincreasing the total space occupied by the lift truck behind the carriervehicle, this distance corresponding to the depth of the structurerequired for the “sideshift” mechanism. This increases the length of theoverall assembly being transported, including both the carrier vehicleand the lift truck being transported. This system also has thedisadvantage of reducing the useful load of the carrier vehicle sincethe moment load applied by the lift truck is increased by an amountcorresponding to this distance. The useful load of the carrier vehicleis also further reduced by an amount corresponding to the additionalweight of the “sideshift” structure. Furthermore, the front liftingtines being separated from the lifting device by a distancecorresponding to the depth of the “sideshift” structure has thedisadvantage in that it reduces the lift truck's useful lift capacitywhen in a static loading position since there is a resultant shift inthe load's centre of gravity due to this added distance being appliedforward of the front wheels. The lift truck's useful lift capacity isagain further reduced by an amount corresponding to the additionalweight of the “sideshift” structure being also applied forward of thefront wheels when in a static loading position.

Another inherent disadvantage of prior art lift trucks of the type thatutilize a telescopic lifting arm articulated about a horizontal axis asthe primary lifting device is that the load follows a convex arc as itis being raised from the ground level to its maximum lifting height, orvice versa. This in turn tends to move the load's centre of gravityforward thereby reducing the maximum rated lifting capacity of the lifttruck. The consequence of this is apparent. Without the lift truckoperator intervening and retracting the telescoping arm, the likelihoodof the lift truck toppling is high.

In general, all lift trucks of the prior art, capable of beingtransported on carrier vehicles, in order to maintain a minimal overhangwhen mounted and transported on the back of a trailer or truck have thedisadvantage in that the lift truck operator safety cage isintentionally kept short which results in the rear portion of theoperator's seat being outboard of or projecting past the rear of thelift truck and operator safety cage. This obviously limits the amount ofprotection that the back of lift truck operator safety cage can provideto the operator which consequently leaves the operator especially pronefrom the rear.

All lift trucks of the prior art, capable of being transported oncarrier vehicles, in general, have the disadvantage in that they do notprovide an automatic method for positively captivating the lift truckwhen mounted and transported on the back of a trailer or truck.

The object of the invention is to remedy the aforementioneddisadvantages of the prior art by providing an improved self propelledlift truck, that is compact and lightweight, and is capable of beingeasily and quickly transported attached onto the back of a carriervehicle.

The invention is particularly defined in the appended claims 1-13 whichare incorporated into this description by reference.

The present invention provides a new and improved lift truck design thatis advantageously more stable and lightweight, having an extremely lowunladen weight relative to its load carrying capacity. It is compact,easily transportable, and is also simple to operate and maintain.

This invention involves a self propelled transportable type lift truckhaving a generally U-shaped frame formed by two spaced parallellongitudinal front members and a rear transverse member, a substantiallyA-shaped gantry structure having two laterally spaced upright assembliesmounted centrally and rearwardly of the U-shaped frame, at least twofront wheels and at least one steerable rear wheel, such that thelifting device pivotally mounted to the A-shaped gantry structurethereby extends from a most retracted position between the longitudinalmembers of the U-shaped frame.

The lifting device to raise and/or lower the lifting tines comprises atelescopic lifting arm having at least one slidable section, thetelescopic arm lifting device mounted between the two laterally spacedupright assemblies of the A-shaped gantry structure such that byactivating a lifting cylinder, the telescopic arm lifting device isarticulated about a horizontal axis (pivoting axis) substantiallyperpendicular to the longitudinal axis of the lift truck.

An operator's cab is located to one side of the telescopic arm liftingdevice and the A-shaped gantry structure; the motive drive unit islocated centrally on the rear of the lift truck's U-shaped frame,adjacent the operator's cab and housed beneath the telescopic armlifting device in an area substantially between the two laterally spacedupright assemblies of the A-shaped gantry structure. When the telescopicarm lifting device is in its lowered and retracted position, as duringthe transport of a load, the telescopic arm lifting device is positioneddirectly over the motive power unit and tucked in an advantageous mannersuch that the field of view from the operator's station is uninhibited.

The telescopic arm lifting device horizontal pivoting axis is centrallylocated aft and above both the steerable rear wheel and the motive powerunit thus maximizing the counterbalance advantage of the lifting devicewhile simultaneously improving the rear stability of the lift truck.

The present invention's telescopic arm lifting device horizontalpivoting axis is advantageously pivotable about a vertical axis so as tovary the yaw angle of the telescopic arm lifting device, therebyproviding a means for shifting laterally the tine carriage and thuseffectively “sideshifting” the load. Furthermore, the vertical axisabout which the telescopic arm lifting device pivots is approximatelylocated centrally and rearwardly of the U-shaped frame. It will beappreciated that since the lateral shift of the lift truck's centre ofgravity is minimized by utilizing this preferred embodiment, theresultant increased stability of the lift truck enhances the operatingrange of the lift truck as well as the safety of the lift truck when“sideshifting” a load. In addition, the yaw control has, on demand, thecapability to automatically self centre the telescopic arm liftingdevice. This simplifies the input required by the operator to centre thetelescopic arm lifting device parallel to and along the lift truck'slongitudinal axis thereby again maximizing the stability and safety ofthe lift truck. Furthermore, it has the added benefit of simplifying themounting process of the lift truck onto the back of a carrier vehicle,since once the telescopic arm lifting device has been automaticallyself-centred, no further adjustment is required of the operator toensure that the telescopic arm lifting device is in its correct positionfor mounting the lift truck onto its support structure on the back of acarrier vehicle.

The invention further provides a new and advantageous telescopic armlifting device arrangement whereby a mechanism is provided which enablesan approximate vertical straight line path for the load as it is raisedor lowered thereby preventing the inadvertent toppling of the lift truckwhen lifting its maximum rated load.

The telescopic arm lifting device supports, on its end farthest from itspivoting axis, a tine carriage capable of rotating about a horizontalaxis under the action of a hydraulic actuator. The tine carriage in turnsupports the lifting tines.

The tine carriage supporting the lifting tines is also automaticallysubjected to the action of a slave cylinder, a fluid displacementlevelling system, which substantially maintains the tines' attitude inthe position from whence it began as the telescopic arm lifting deviceis raised or lowered.

The lift cylinder for raising and lowering the telescopic arm liftingdevice, and the master cylinder for controlling the action of the slavecylinder are mounted either side of the telescopic arm lifting device.

The lift truck of the present invention provides a steerable rear wheelcapable of 180 degree steering for tight turning radii as well as forreducing rear wheel overhang of the lift truck while being transportedon the back of a carrier vehicle.

Advantageously, the steering mechanism of the present invention thatprovides 180 degree steering capability of the steerable rear wheelautomatically self adjusts itself to compensate for any wear and tear ofthe steering mechanism that may occur while operating the lift truck.Consequently, the operator of the lift truck always achieves positiveand direct control of the steerable rear wheel thereby further enhancingthe safe operation of the lift truck.

The steering mechanism of the present invention advantageously has avery compact and shallow profile and by virtue of its very designenables the motive power unit to be disposed in a housing substantiallybetween the two laterally spaced upright assemblies comprising theA-shaped gantry structure, above the steerable rear wheel and below thetelescopic arm lifting device thereby keeping the overall envelopeheight of the motive power unit housing and the telescopic arm liftingdevice above the steerable rear wheel to a very minimum thus enablingthe operator to have an uninhibited field of view when transporting aload. Consequently, the safe operation of the lift truck is furtherenhanced.

The extremities of the longitudinal members of the U-shaped framesupports inclined telescoping stabilizers (outriggers), having at leastone slidable section, capable of firm contact with the underlying groundsurface in front of the front wheels. The outriggers stabilize the lifttruck when lifting a load when the telescopic arm lifting device isextended from a most retracted position to a position forward of thefront wheels.

The invention further provides a complementary means that enables thelift truck to both displace from and retract back into the rear of thecarrier vehicle a support structure forming a carrier surface for thelift truck and a latching mechanism that automatically and positivelycaptivates the lift truck to the support structure carrier surface whenmounted on the rear of a carrier vehicle.

The lift truck, in accordance with the invention, is intended to betransportable on the back of a carrier vehicle and in view of applicabletransportation regulations as well as concerns for safety and thedynamic effects of a trailing load, the unladen weight and the overalloverhang length of the lift truck when mounted on the back of thecarrier vehicle are kept to a minimum. The latter is achieved withoutforeshortening or compromising the length of the operator safety cagethereby still maximizing the amount of protection offered to theoperator from the rear.

The lift truck design disclosed herein has the additional advantage ofproviding relatively large ground clearances under the U-shaped frameand the telescopic arm lifting device, as well as under the liftingtines, when transporting a load. This enables the lift truck to travelover rough and irregular terrain, as well as to negotiate street curbsor other obstacles encountered while operating in an urban environment.

Further characteristics and advantages of the lift truck according tothe invention will become clear in the course of the detaileddescription which follows with reference to the appended drawings,provided by way of non-limiting example, in which:

FIG. 1 is a perspective view of the lift truck with the telescopic armlifting device in the lowered and retracted position and the liftingtines in the reclined position as would be typical when the lift truckwould be transporting a load;

FIG. 2 is a side view of the lift truck with the operator and shown inthree positions, namely in a ground contact position with the telescopicarm lifting device fully extended, in a raised position with thetelescopic arm lifting device fully retracted, and in a raised positionwith the telescopic arm lifting device fully extended;

FIG. 3 is a side view of the lift truck with the operator with thetelescopic arm lifting device in the lowered and retracted position andthe lifting tines in the reclined position as would be typical when thelift truck would be transporting a load;

FIG. 4 is a rear view of the lift truck with the telescopic arm liftingdevice in the lowered and retracted position and showing the location ofthe motive power unit and pumps;

FIG. 5 is a front view of the lift truck with the telescopic arm liftingdevice in the lowered and retracted position and the lifting tines inthe reclined position as would be typical when the lift truck would betransporting a load;

FIG. 6 is a plan view of the lift truck with the telescopic arm liftingdevice in the lowered and retracted position and the lifting tines inthe reclined position as would be typical when the lift truck would betransporting a load;

FIG. 7 is a plan view of the lift truck's telescopic arm lifting devicein an extended centred position and in an extended yawed positionindicating lateral movement of the lifting tines;

FIG. 8 is an exploded perspective view of a portion of the A-shapedgantry structure having two laterally spaced upright assemblies whichsupports the main lifting arm indicating the ancillary components of theyaw mechanism and the location of the motive power unit;

FIG. 9 is a load diagram for the lift truck without the use of theinclined telescoping stabilizers;

FIG. 9a is a load diagram for the lift truck using inclined telescopingstabilizers;

FIGS. 10, 10 a and 10 b illustrate the sequence for mounting the lifttruck onto the back of a carrier vehicle;

FIG. 11 shows the automatic captivation mechanism for locking the lifttruck onto the carrier support structure when mounted and transported onthe back of a truck or trailer;

FIG. 12 is a perspective view of the lift truck steering mechanism;

FIG. 13 is a rear view of the lift truck showing the low profile of thesteering mechanism and also further indicates the location of the motivepower unit;

FIG. 14 is the hydraulic control circuit for the telescopic arm liftingdevice's approximate straight line lift;

FIG. 15 is the electrical control circuit for the telescopic arm liftingdevice's self centering mechanism;

FIG. 16 is an enlarged partial side view of the lifting tines, the tinecarriage, the tine carriage pivoting axis, the actuating and slavecylinder, and the forward end of the telescopic arm; and

FIG. 17 is the hydraulic control circuit for the fluid displacementlevelling system for the tine carriage.

FIG. 1 illustrates a perspective view of an assembled lift truck,designated generally as A, which is the subject of the presentinvention.

Referring to FIGS. 1 through 8, the preferred embodiment of the presentinvention is shown. Lift truck A is preferably propelled withhydraulically actuated front drive wheels 5 and 6, and a steerable reardrive wheel 7. The front drive wheels 5 and 6 are located at theextremities of the longitudinal members of the U-shaped frame 1. Thesteerable rear wheel 7 is installed approximately in the centre of andrearward of the main body of a generally U-shaped frame 1. A motivepower unit 3 is centrally located at the rear of the U-shaped frame 1and the operator's station 2 is located to one side of the motive powerunit 3. At the other side of the motive power unit 3 is a clear areawhich in the present embodiment preferably accommodates, but is notlimited to, a spare wheel 18.

The steerable rear wheel 7 is controlled by a steering wheel 8 locatedin the operator's station 2, as are all system functions for controllingthe lift truck A which are easily accessible to the operator 23.

Hydraulic power is preferably provided to the drive wheels 5, 6, and 7by a double acting variable displacement hydraulic pump 50. A motivepower unit 3 is preferably a unit comprising an internal combustionengine. The motive power unit 3 is located centrally on the rear of theU-shaped frame 1, above the steerable rear wheel 7, below the mainlifting arm 4, and adjacent to the operator's station 2. The motivepower unit 3 drives the variable displacement hydraulic pump 50, as wellas the necessary distribution and control elements that are includedwith the variable displacement hydraulic pump 50.

The main lifting arm 4 for lifting the tine carriage 11 and thus thetines 13 comprises a telescopic arm 9 and is pivotally mounted to asubstantially A-shaped gantry structure having two laterally spacedupright assemblies 30 and 32. The main lifting arm 4 is locatedcentrally on the rear of the U-shaped frame 1, between the two laterallyspaced upright assemblies 30 and 32 and above the steerable rear wheel7. The motive power unit 3 is disposed in an area substantially betweenthe two laterally spaced upright assemblies 30 and 32 and below the mainlifting arm 4. The main lifting arm 4 is raised and lowered by ahydraulic lift cylinder 14 which is at one side of the main lifting arm4. A fluid displacement master cylinder 15 is located at the other sideof the main lifting arm 4. A further cylinder (not shown) is locatedwithin the telescopic arm 9 for extending and retracting the telescopicarm 9.

The operator's station 2 is located on one side of the U-shaped frameadjacent to the A-shaped gantry structure upright assembly 30, the mainlifting arm 4, the telescopic arm 9, and the motive power unit 3.

Referring to FIG. 8, the main lifting arm 4 pivots about a substantiallyhorizontal axis on shaft 10 which is substantially perpendicular to thelongitudinal axis of the lift truck A. Shaft 10 is supportedapproximately aft and above both the steerable rear wheel 7 and themotive power unit 3 and by the A-shaped gantry structure laterallyspaced upright assemblies 30 and 32. The gantry upright assembly 30 isprovided with a spherical bearing 33 which is seated in hole 39.Spherical bearing 33 engages one end of the shaft 10. An elongated slideblock 37 is located behind the elongated slot 35 on gantry uprightassembly 32. The other end of the shaft 10, which is the end nearest theturned down end 36 of shaft 10 is seated in hole 40 of the slide block37. One end of hydraulic cylinder 16 is connected to the end 36 of theshaft 10, the other end is connected to gantry upright assembly 32.Extending fully the hydraulic cylinder 16 yaws the main lifting arm 4about a substantially vertical axis,“V” and thus the tines 13 areshifted laterally a distance “d”, as depicted in FIG. 7, to the right ofthe longitudinal axis of the lift truck A. Conversely, retracting fullythe hydraulic cylinder 16 shifts the tines 13 laterally approximatelythe same distance left of the longitudinal axis of the lift truck A.Shaft 10 further contains two thrust washers 38 which are sandwichedbetween the outer surfaces of main lifting arm 4 and the inner surfacesof the two gantry upright assemblies 30 and 32. Thrust washers 38mitigate any possible interference the main lifting arm 4 may cause uponthe two gantry upright assemblies 30 and 32 when yawing the main liftingarm 4 a distance “d” either side of the longitudinal axis of the lifttruck A. The vertical axis about which the main lifting arm 4 yaws isapproximately located centrally and rearwardly of the U-shaped frame 1.

The present invention provides an electro-hydraulic control system, asdepicted in FIG. 14, in which the telescopic arm 9 of the main liftingarm 4 is automatically retracted and extended as the main lifting arm 4is raised and lowered. Load diagrams for the lift truck A are shown inFIGS. 9 and 9a. FIG. 9 shows the load curve carrying capacity of thelift truck A without the telescoping stabilizers 17 being used. FIG. 9ashows the load curve carrying capacity of the lift truck A with thetelescoping stabilizers 17 being used. Each figure has four load curvesdepicted. Load curve 19 indicates the travel path that the load centrefollows when the telescopic arm 9 is fully retracted and the mainlifting arm 4 is raised or lowered and the electro-hydraulic controlsystem is not active. Load curve 22 indicates the travel path that theload centre follows when the telescopic arm 9 is fully extended and themain lifting arm 4 is raised or lowered and the electro-hydrauliccontrol system is not active. Load curve 20 indicates the travel paththat the load centre follows when the telescopic arm 9 is initiallyfully extended and the main lifting arm 4 is raised from its lowestposition to its highest and the electro-hydraulic control system isactive. Load curve 21 indicates the travel path that the load centrefollows when the telescopic arm 9 is initially fully extended and themain lifting arm 4 is lowered from its highest position to its lowestand the electro-hydraulic control system is active. As the load curve 22clearly demonstrates, when a load is being raised from ground level toits maximum lifting height, or vice versa, the load centre movesdramatically forward thereby significantly reducing the rated liftingcapacity of the forklift truck A. To overcome this inherent problem, anelectro-hydraulic control system as depicted in FIG. 14, is providedwhich automatically retracts and extends the telescopic arm 9 as themain lifting arm 4 is raised or lowered obviating any need from the lifttruck operator 23 to intervene to keep the load centre within the safeoperating limits of the lift truck A; 2500 kg [5500 lbs] being themaximum rated lifting capacity of the lift truck A with the telescopingstabilizers 17 in use.

From FIG. 14, it can be seen that the directional control valve 52controls the action of the main lifting arm 4 lift cylinder 14 and thetelescopic arm 9 telecoping cylinder 58. Directional control valve 52 iscontrolled by the operator 23 from the operator cab 2. Oil is pumped bya gear pump 51 through a return filter 59 to the oil containment tank 60until either side 52 a or side 52 b of directional control valve 52 isactivated. Oil flow through side 52 a of directional control valve 52extends the lift cylinder 14 which in turns raises the main lifting arm4. Oil flow through side 52 b of directional control valve 52 retractsthe lift cylinder 14 which in turns lowers the main lifting arm 4. Flowdivider 53 splits inlet oil flow into two discrete output volumes basedon a predetermined fixed ratio setting of flow divider 53. One oil flowpath from flow divider 53 is directed towards the telescopic armcylinder 58, the other is directed toward the lift cylinder 14. Betweenflow divider 53 and telescopic arm cylinder 58 is a solenoid operatedtwo position valve 54. Its action is directly controlled by the movementof the main lifting arm 4. At a predetermined position of the mainlifting arm 4, solenoid valve 54 alternates the side through which oilwill flow. Valves 6 are unloading valves which enable the free flow ofoil to the lift cylinder 14 should the telescopic arm cylinder 58 bottomout before the lift cylinder 14 does. Valves 57 are load holding valves,and are known per se, which prevent a load from being dropped in theevent of a hose failure. Valves 55 control the speed at which oil canexit from the lift cylinder 14 and therefore control the ascent ordescent of the main lifting arm 4.

In the operation of the lift truck A, a typical sequence of events couldoccur as follows: When raising the main lifting arm 4 from its lowestpoint to its highest, oil flows from side 52 a of valve 52. This oil isthen split into two proportional amounts by the flow divider 53. Oneflow path is directed toward the lift cylinder 14, the other toward thetelescopic cylinder 58. Oil flowing to the lift cylinder 14 free flows,without restriction, through the speed control valve 55 and the loadholding valve 57 into the bore side 14 b of the lift cylinder 14. Thelift cylinder 14 then extends and the main lifting arm 4 is raised.

During this motion of raising the main lifting arm 4, the second oilflow from the flow divider 53 passes through side 54 a of valve 54 intothe annular side 58 a of the telescopic cylinder 58. This has the effectof retracting the telescopic arm 9 as the main lifting arm 4 is beingraised.

Exit oil from both the lift cylinder 14 and the telescopic cylinder 58recombine at the flow divider 53 and is returned to the oil containmenttank 60 as a single flow of oil. Exit oil flow of the lift cylinder 14is also regulated by valve 55 to optimize the speed at which the liftingarm 4 ascends to match the movement of the telescopic arm 9.

At a set attitude position of the main lifting arm 4, oil that wasflowing through side 54 a of valve 54 is directed to flow through side54 b and into the bore side 58 b of the telescopic cylinder 58. Thischanges the original effect of retracting the telescopic arm 9 whileraising the main lifting arm 4 to extending it. Summarizing then thisaction; as the main lifting arm 4, with the telescopic arm 9 fullyextended, is raised from its lowest position to its highest, thetelescopic arm 9 alternates between retracting and extending, thealternating action occurring at a set attitude position of the mainlifting arm 4. This is evidenced in FIGS. 9 and 9a by load curves 20.This control is available also when lowering the load from its highestposition to its lowest and any combination thereof.

The present invention further provides an electro-hydraulic controlmechanism, as depicted in FIG. 15, for automatically self centering themain lifting arm 4 parallel to and along the longitudinal axis of thelift truck A. When the main lifting arm 4 is in its centred position,indicating lights 72 and 76 are unlit, limit switches 71 and 75 are intheir normally open position, control relay coils 65, 70 and 74 arede-energized, control relay contacts 66, 67 and 68 are open, andsolenoids 69 and 73 are de-energized. Solenoids 69 and 73 activate anopen centre spring-returned directional control valve, and when bothsolenoids are de-energized, hydraulic fluid passes freely through thevalve to the hydraulic fluid containment tank 60. The open centrespring-returned directional control valve controls the Yaw cylinder 16which in turn controls the yaw of the main lifting arm 4.

Should the operator 23 have a reason to yaw the main lifting arm 4 leftof centre of the longitudinal axis of the lift truck A, limit switch 71closes, indicating light 72 comes on, control relay coil 70 is energizedand remains energized while the limit switch 71 is closed. When controlrelay coil 70 is energized, it latches all control relay contacts 67. Toautomatically return the main lifting arm 4 back to its home or centredposition, the operator 23 presses the momentary push-button 64. Sincecontrol relay contacts 67 are already latched, pressing the momentarypush-button 64 energizes control relay coil 65, latching all controlrelay contacts 66, thereby preventing the circuit from disconnectingwhen the push-button 64 is released after momentary contact. Withcontrol relay contacts 66 and 67 latched, solenoid 69 is energized,activating the open centre spring-returned directional control valvethat controls the Yaw cylinder 16, thereby yawing the main lifting arm 4right and back to its home or centred position. Once the main liftingarm 4 reaches its home position, limit switch 71 opens, indicating light72 goes off signifying that the main lifting arm 4 is centred, andcontrol relay coil 70 is de-energized. De-energizing control relay coil70 unlatches all control relay contacts 67 thereby breaking the circuitto solenoid 69 and the control relay coil 65. With the circuit broken tocontrol relay coil 65, all control relay contacts 66 unlatch. Withsolenoid 69 de-energized, hydraulic fluid again passes freely throughthe open centre spring-returned directional control valve to thehydraulic fluid containment tank 60 and yawing of the main lifting arm 4is stopped. The same control is available to the operator 23 when themain lifting arm 4 is yawed right of centre of the longitudinal axis ofthe lift truck A.

The telescopic arm 9 supports, on its end farthest from its pivotingaxis 10, a tine carriage 11 capable of rotating about a horizontal axis77, as shown in FIG. 16, under the action of a hydraulic actuator 78.The tine carriage 11 in turn supports the lifting tines 13.

The tine carriage 11 supporting the lifting tines 13 is subjected to thecombined action of a master cylinder 15 and a slave cylinder 78, theircombination creating a fluid displacement levelling system as depictedby FIG. 17, which substantially maintains the tines' 13 attitude in theposition from whence they began as the telescopic arm 9 is raised orlowered. Further describing the fluid displacement levelling system, oneend of the slave cylinder 78 is connected to the tine carriage 11 whilethe other end is connected to the telescopic arm 9. One end of themaster cylinder 15 is connected to one side of the main lifting arm 4while the other end is connected to the U-shaped frame 1. One end of thelift cylinder 14 for raising and lowering the main lifting arm 4 isconnected to the other side of main lifting arm 4 while the other end isconnected to the U-shaped frame 1. As the lift cylinder 14 is extended,the main lifting arm 4 is raised. As the main lifting arm 4 is raised,it forces the extension of the master cylinder 15 thereby displacinghydraulic fluid from the annular side 15 a of the master cylinder 15 andforcing it into the annular side 78 a of the slave cylinder 78. Thisthen in turn forces oil out of the bore side 78 b of the slave cylinder78 back into the bore side 15 b of the master cylinder 15. This actioncontinues as the main lifting arm 4 is raised. Since the tine carriage11 is supported at the end of the telescopic arm 9 and is furthercapable of rotating about a horizontal axis 77, as a result of the fluiddisplacement action of the master cylinder 15 and the slave cylinder 78,the attitude of tine carriage 11, and thus the lifting tines 13 aresubstantially maintained in the position from whence they began as themain lifting arm 4 is raised. The same is true when the main lifting arm4 is lowered except the hydraulic oil flow paths are reversed.

The lift truck A of the present invention provides a steerable rearwheel 7 capable of 180 degree steering. The steering mechanism,designated generally as B, as depicted by FIGS. 12 and 13, that provides180 degree steering capability of the steerable rear wheel 7automatically self adjusts itself to compensate for any wear and tear ofthe steering mechanism B that may occur while operating the lift truckA. Furthermore, the steering mechanism B, as depicted by FIG. 13, has avery compact and shallow profile and by virtue of its very designenables the motive power unit 3 to be disposed in a housing above thesteerable rear wheel 7 and below the main lifting arm 4. The steeringmechanism B comprises two hydraulic cylinders 46 a and 46 b which arecontrolled from the operator cab 2 by steering wheel 8, two tensionvalves 47, one being fixedly attached to the housing of each cylinder 46a and 46 b, a simplex chain 48 with one end of the chain 48 beingattached to hydraulic cylinder 46 a and the other end being attached tohydraulic cylinder 46 b, and a load bearing sprocket assembly 49. Theload bearing sprocket assembly 49 has two independent load bearingparts, one being attached to the lift truck U-shaped frame 1, the otherto the rear steerable wheel 7, their combination enabling the relativemovement to each other about a common substantially vertical pivotingaxis when acted upon by the simplex chain 48 which engages the loadbearing sprocket assembly 49. Movement of the steerable rear wheel 7 iseffected by the simplex chain 48 being wrapped around the load bearingsprocket assembly 49 and being alternately pulled by hydraulic cylinder46 a and hydraulic cylinder 46 b. Two cylinders are used on the presentinvention instead of opting for other configurations that could use onecylinder because it allows the operator 23 to have the same sense offeeling at the steering wheel 8 when turning the steerable rear wheel 7left or right; the two cylinder configuration utilizes equal areas ofeach cylinder whereas the one cylinder would have to utilize unequalareas of the cylinder. When moving the lift truck A forward and turningto the right, cylinder 46 a retracts which forces the extension ofcylinder 46 b and in doing so, rotates the load bearing sprocketassembly 49 counterclockwise. Tension in the simplex chain 48 ismaintained at all times by valve 47 on cylinder 46 b during this motion.To move the lift truck A forward and to the left, the procedure isreversed. Since tension of the simplex chain 48 is always maintainedwith this configuration, wear of the teeth of the load bearing sprocketassembly 49, and wear and stretching of the simplex chain 48 during theoperating life of lift truck A is automatically compensated for.Consequently, the operator 23 of the lift truck A always achievespositive and direct control of the steerable rear wheel 7 which furtherenhances the safe operation of the lift truck.

The extremities of the longitudinal members of the U-shaped frame 1supports inclined telescoping stabilizers 17 having at least oneextendible tubular section, capable of firm contact with the underlyingground surface in front of the front wheels. The advantages of utilizingtelescoping stabilizers 17 is clearly evidenced and self explanatory inFIGS. 9 and 9a, in which FIG. 9 shows the load curve carrying capacityof the lift truck A without the telescoping stabilizers 17 being usedand FIG. 9a shows the load curve carrying capacity of the lift truck Awith the telescoping stabilizers 17 being used.

The present invention provides a means, as depicted by FIGS. 10, 10 a,and 10 b for supporting and positively captivating itself when mountedand transported on the back of a carrier vehicle 41. The lift truckoperator 23, without dismounting from the lift truck A, canautomatically displace from the rear of the carrier vehicle 41 a supportstructure 43 forming a carrier surface 44 for the lift truck A by firstextending the telescopic arm 9 approximately 280 mm [11 inches] and theninserting the tines 13 of the lift truck A into tine support sleeves 42which are fixedly attached to the carrier vehicle 41 thereby engaging adevice which in turn moves the support structure 43 into place.

Once the support structure 43 is in place, as depicted in FIG. 10, inorder to mount the lift truck A onto the support structure carriersurface 44, a combined movement is effected that includes both theraising and pivoting of the lift truck A about the tine carriagehorizontal pivoting axis 77, until the position is reached in which thelift truck A is completely lifted off the ground, and is slightlyinclined forward, at an angle between 9 and 10 degrees. The telescopicarm 9 is then completely retracted, which has the effect of resting theU-shaped frame 1 of the lift truck A on the support structure carriersurface 44 as depicted in FIG. 10a. Preferably, the steerable rear wheel7 is locked in position, either completely to the left or to the right,as depicted by FIG. 10b, so as to minimize the space occupied by thelift truck A behind the carrier vehicle 41.

The lift truck is provided with captivating means which enable the lifttruck operator 23 to automatically captivate and lock the lift truck A,as depicted by FIGS. 10, 10 a, and 11, onto the platform supportstructure carrier surface 44 without first dismounting from the lifttruck A. Projecting from the platform support structure carrier surface44 are two laterally spaced male stub shaft latching mechanisms 45. Thelatching mechanism 45 is comprised of a latching lever with a leadingedge 60, a pin 62 about which the latching lever 60 pivots, a tensionspring 61 for maintaining the latching lever 60 in its latched position,and finally a handle 63 for manually disengaging the latching lever 60.As the U-shaped frame 1 of the lift truck A begins to rest on thesupport structure carrier surface 44, the male stub shaft latchingmechanism 45 engages a mating female receptacle which is fixedlyattached to the U-shaped frame 1. The mating female receptacle isallowed to slip down over the male stub shaft latching mechanism 45 bythe leading edge of the latching lever 60 which withdraws into theinterior of the male stub shaft 45 as the lift truck A begins to rest onthe support structure carrier surface 44. When the lift truck A is fullyrested on the support structure carrier surface 44, the latching lever60 is again projected from the interior of the male stub shaft 45 andmaintained in this position by the tension spring 61 until manuallydisengaged by pushing down on handle 63. When in this latched position,the mating female receptacle is prevented from moving back up the malestub shaft 45 thereby effectively locking the lift truck A onto thesupport structure carrier surface 44.

To dismount the lift truck A from the back of the carrier vehicle 41,the operator 23 must first push down on handle 63 thereby disengagingthe latching lever 60. The latching lever 60 is maintained in thedisengaged position by a trip-lock latch. Once disengaged, the lifttruck A can be freely removed from the support structure carrier surface44. Also, when the lift truck A is removed from the back of the carriervehicle 41, the present invention provides a means for automaticallyretracting the lift truck support structure 43 back into the carriervehicle 41 by removing the tines 13 of the lift truck A from the tinesupport sleeves 42 which are fixedly attached to the carrier vehicle 41.Once the lift truck support structure 43 is fully retracted, thetrip-lock latch for maintaining the latching lever 60 in its disengagedposition is reset. The latching mechanism described is the preferredembodiment from an economic standpoint. However other less economicvariants of the above which provide latching and unlatching controldirectly from the operator cab 2 are also available with the presentinvention. Briefly, the two laterally spaced male stub shaft latchingmechanisms 45 can instead be fixedly attached to the U-shaped frame 1and the mating female receptacles can be incorporated into the platformsupport structure carrier surface 44. The tension spring 61 can then besubstituted for an electric, an electro-hydraulic, or hydraulic controlmechanism which latches on turning off the lift truck A and unlatcheswhen turning it on.

The lift truck A design disclosed herein provides relatively largeground clearances “h”, as depicted by FIG. 3, under the U-shaped frame 1and the main lifting arm 4, as well as under the lifting tines 13enabling the lift truck A when transporting a load to travel over roughand irregular terrain, as well as to negotiate street curbs or otherobstacles encountered while operating in an urban environment.

It is to be understood that the present invention is not limited to thespecific details described above which are given by way of example onlyand that various modifications and alterations are possible withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A materials handling lift truck capable of beingattached onto the back of a carrier vehicle, said truck having agenerally U-shaped frame formed by two spaced apart parallellongitudinal members and a rear transverse member, a motive power unit,two front wheels, a steerable rear wheel, an operator's station, and alifting device comprising a telescopic arm, having at least one slidablesection and a lift cylinder, mounted such that, by activating the liftcylinder, the telescopic arm pivots about a relatively horizontal axiswhich is located approximately centrally and rearwardly of the U-shapedframe; and wherein the operator's station is located on one side of thelift truck and the motive power unit is located approximately centrallyat the rear of the U-shaped frame adjacent to the operator's station,such that when the telescopic arm is in a retracted and loweredposition, as in the transport of a load, or attached onto the back of acarrier vehicle, it is approximately centrally located above both therear steerable wheel and the motive power unit and adjacent theoperator's station.
 2. A materials handling lift truck as claimed inclaim 1, further comprising a substantially A-shaped gantry structurehaving two laterally spaced upright assemblies that are immovablerelative to the frame and between which the telescopic arm liftingdevice is pivotally mounted, the A-shaped gantry structure being locatedapproximately centrally and rearwardly of the U-shaped frame,approximately above the rear steerable wheel, and the motive power unitbeing disposed in an area substantially between the two laterally spacedupright assemblies and below the pivotally mounted telescopic armlifting device.
 3. A materials handling lift truck as claimed in claim1, wherein the slidable section of the telescopic arm lifting device isautomatically retractable and extendable as the lifting device is raisedand lowered.
 4. A materials handling lift truck as claimed in claim 1,further comprising a tine carriage and lifting tines supported by thetine carriage, the tine carriage being rotatably moveable in a verticalplane while being supported at the end farthest from the telescopic armlifting device substantially horizontal pivoting axis, and having anautomatic fluid displacement leveling system which substantiallymaintains the attitude of the tine carriage and the lifting tines intheir original orientation as the telescopic arm lifting device israised or lowered.
 5. A materials handling lift truck as claimed inclaim 4, including a master cylinder for controlling the action of theautomatic fluid displacement leveling system and wherein the liftcylinder for raising and lowering the telescopic arm lifting device, andthe master cylinder are mounted either side of the telescopic armlifting device.
 6. A materials handling lift truck as claimed in claim1, further comprising a control mechanism for self centering thetelescopic arm lifting device parallel to and along the lift truck'slongitudinal axis.
 7. A materials handling lift truck as claimed inclaim 1, wherein the steerable rear wheel is capable of being turnedthrough 180 degrees.
 8. A materials handling lift truck as claimed inclaim 7, further comprising a steering mechanism that provides 180degree steering capability of the steerable rear wheel which is selfadjustable to compensate for any wear and tear of the steering mechanismthat may occur while operating the lift truck.
 9. A materials handlinglift truck as claimed in claim 8, wherein the steering mechanism has acompact and shallow profile thereby enabling the motive power unit to bedisposed in a housing above the steerable rear wheel and below thetelescopic arm lifting device.
 10. A materials handling lift truck asclaimed in claim 8, wherein the steering mechanism comprises a loadbearing sprocket assembly having two independent load bearing parts, onebeing attached to the lift truck U-shaped frame, the other to the rearsteerable wheel, the load bearing parts enabling relative movement toeach other about a common substantially vertical pivoting axis whenacted upon by a chain which engages the load bearing sprocket assembly.11. A materials handling lift truck as claimed in claim 1, furthercomprising inclined telescoping stabilizers, having at least oneextendible tubular section, supported at the extremities of longitudinalmembers of the U-shaped frame which are capable of firm contact with theunderlying ground surface in front of the front wheels.
 12. A materialshandling lift truck according to claim 1, further comprising captivatingmeans for supporting and locking the lift truck onto the back of acarrier vehicle, whereby the lift truck operator, without firstdismounting from the lift truck, can engage and extend from the carriervehicle a platform support structure for the lift truck, and cancaptivate the lift truck onto the platform support structure.
 13. Amaterials handling lift truck capable of being attached onto the back ofa carrier vehicle, the lift truck comprising a generally U-shaped frame;a substantially A-shaped gantry structure having two laterally spacedupright assemblies located approximately centrally and rearwardly of theU-shaped frame; a motive power unit disposed in an area substantiallybetween the two laterally spaced upright assemblies of the A-shapedgantry structure; two front wheels; a steerable rear wheel; a telescopicarm lifting device having at least one slidable section, rotatablymounted by a shaft between the two laterally spaced upright assembliesof the A-shaped gantry structure such that by activating a lift cylinderthe telescopic arm lifting device pivots about a relatively horizontalaxis located approximately centrally and rearwardly of the U-shapedframe and approximately above the motive power unit and the rearsteerable wheel; lifting tines that are mounted on a tine carriage thatis rotatably moveable in a vertical plane and supported at the endfarthest from the telescopic arm lifting device substantially horizontalpivoting axis; an operator's station located on one side of the U-shapedframe and adjacent to the A-shaped gantry structure, wherein the shaftis mounted to the spaced upright assemblies for pivoting about asubstantially vertical axis and relative to the spaced uprightassemblies so that the substantially horizontal pivoting axis of thetelescopic arm lifting device is capable of being pivoted about thesubstantially vertical axis so as to precisely vary the yaw angle of thetelescopic arm lifting device thereby enabling the accurate lateralmovement of the lifting tines within a predetermined distance.